1. Field of the Invention
The present invention relates to electrochromic devices which exhibit coloration and bleaching thereof at ambient temperature by control of the polarity of an induced electric field. More particularly, this invention relates to electrochromic devices wherein a layer of cathodic electrochromic material comprising non-stoichiometric, oxygen deficient metal oxide is provided by pyrolytic deposition techniques.
2. Discussion of the Related Art
Electrochromic devices are devices in which a Physical/chemical change produced in response to the induced electric field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., UV, IR, and visible radiation. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material 12 and an ion-conductive insulating layer 14 which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers, 16 and 18 in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. The electrode layers, 16 and 18 in FIG. 1, are provided on substrates, 20 and 22 of FIG. 1, which substrates may be of a material such as glass. The combination described is provided with external electrical means for applying a voltage to the electrodes to cause coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colored state or from the colored state to the bleached is termed "switching". The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "memory". Electrochromic devices of this type have been described for several uses, such as image display, for light filtering, etc.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. The electrochromic metal oxide layer has been applied by a number of techniques: vacuum deposition, chemical vapor deposition, thermal evaporation, sputtering, and electron beam evaporation. See, e.g., U.S. Pat. Nos. 4,194,812; 4,278,329; 4,645,308; 4,436,769; 4,500,878; 4,150,879; 4,652,090; 4,505,021; and 4,664,934. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged ion, preferably, a proton or a lithium ion. The electrolyte layer is generally a liquid electrolyte solution which comprises polymers or copolymers containing acidic groups such as polystyrene sulfonic acid or a compound like lithium chloride. The electrolyte layer also may be a gel or a solid material.
One of the problems with the prior art devices of this type is that the electrochromic layers of such devices are not provided by methods which are suitable for coating large areas such as would be necessary if, e.g., sunroofs or windshields of automobiles were to be made as electrochromic devices. As would be apparent, it would be advantageous to make such items electrochromic devices which could be colored to a desired intensity to keep out radiation like UV, IR and visible transmissions at will. For example, it might be desirable to "color" the sunroof and the windows to allow minimum transmittance when the automobile is parked to prevent the interior of the automobile from heating up on a sunny day In another embodiment, the windshield might be colored to an intensity which allows operation of the automobile yet reduces the amount of visible transmission through the windshield.
Present methods for providing the electrochromic layer also are generally incapable of providing an electrochromic layer having sufficiently low transmission of electromagnetic radiation. It is important to provide a device capable of low transmission, particularly of IR radiation, if the device is to be used as the sunroof or windshield of an automobile. Another problem encountered with prior art electrochromic devices is that they lose their ability to switch with time, i.e., after numerous switches the percent of electromagnetic radiation that is transmitted by the electrochromic material in its colored state increases. This is particularly problematic if the device is to be used through many cycles to keep out undesirable radiation, as would be intended by a sunroof or windshield of an automotive vehicle or windows of a building. Still another problem of such devices is that the electrochromic material, if it is coupled with a liquid electrolyte layer, has a tendency to be solvated by the liquid electrolyte layer. This reduces the durability of the device as well as the number of switches through which it can suitably function.
An attempt to improve the resistance of electrochromic material to the degrading effects of the electrolyte is taught is U.S. Pat. No. 4,233,339 to Leibowitz et al. It is disclosed therein that by subjecting thin, electrochromic layers deposited on substrate electrodes to a special heat treatment at a selected high temperature for a selected short time, at least a free portion of each layer is converted from the amorphous to the crystalline form. It is further taught that this outer layer of the electrochromic material significantly increases the resistance of the electrochromic layer to degradation by the liquid electrolyte. In U.S. Pat. No. 4,175,837, to Yano et al, it is disclosed that solvation of a tungsten oxide film can be decreased by forming a WO.sub.3 film on a glass substrate under conditions where the substrate is held at a high temperature, that is, between 250.degree. C-450.degree. C. According to that patent, the WO.sub.3 film is deposited by thermal evaporation and vacuum deposition. It taught that the transparency of the WO.sub.3 film is undesirably lessened, however, when the substrate is held above 450.degree. C. during the formation of the WO.sub.3 film.
It would be desirable if a method could be found to form a durable electrochromic device capable of substantially reducing transmission of electromagnetic radiation, wherein the device is capable of switching for prolonged periods of time without substantially any loss of such electrochromic activity, and wherein the electrochromic layer provided on a surface (i.e., on an electrode) would be resistant to dissolution by the electrolyte. It would be most advantageous if a method for providing such an electrochromic layer would be simple and commercially suitable for coating large areas easily.
The invention disclosed herein is capable of overcoming the aforementioned problems of prior art devices. The invention comprises providing the electrochromic layer by pyrolytic deposition techniques.
Pyrolytic deposition techniques comprises heating a surface and applying a composition at room temperature onto the heated surface. For the sake of convenience, the composition is generally sprayed onto the heated surface. The heat from the hot surface causes chemical degradation of the sprayed composition and subsequent recombination of components of the degraded material with the ambient gas to form a material on the surface. Various U.S. Patents describe the pyrolytic deposition of metal oxides onto glass to change its apparent color or reduce it transmission to electromagnetic radiation. See, e.g., U.S. Pat. Nos. 4,217,392; 4,349,369; and 3,374,156. None of these patents teaches or discloses an electrochromic device wherein the electrochromic layer comprises oxygen deficient metal oxide which has been deposited by pyrolytic deposition as in the present invention.